Solution mining under an inert gas

ABSTRACT

This method includes providing an cased borehole located within a salt bed, injecting an aqueous liquid into the cased borehole at a first pressure, in order to expose the salt bed to the aqueous liquid, thereby dissolving at least a portion of the salt bed and forming a brine solution within an underground storage volume, withdrawing at least a portion of the brine solution from the underground storage volume, and injecting an inert gas into the cased borehole at a second pressure, in order to provide an inert blanket within the underground storage volume, wherein the second pressure that is greater than the first pressure but less than the maximum allowable pressure of the cavern.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 (a) and (b) to US Provisional Patent Application No. 62/081,655 filed Nov. 19, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

Leached storage caverns in salt formations typically have a relatively flat roof. Large diameter flat roofs in a salt cavern can be unstable due to the low tensile strength of the salt, salt movement, fractured salt or low pressure in the cavern. The stability of the roof may be increased by leaching a modified dome shape in the roof.

The leaching of storage caverns in salt formations is typically performed under a blanket of liquid hydrocarbons. Some storage applications may require very clean or ultra pure caverns, where residual hydrocarbons could contaminate the stored product. To prevent these contamination issues, ultra pure salt caverns can be leached under an inert gas blanket.

SUMMARY

This method includes providing an cased borehole located within a salt bed, injecting an aqueous liquid into the cased borehole at a first pressure, in order to expose the salt bed to the aqueous liquid, thereby dissolving at least a portion of the salt bed and forming a brine solution within an underground storage volume, withdrawing at least a portion of the brine solution from the underground storage volume, and injecting an inert gas into the cased borehole at a second pressure, in order to provide an inert blanket within the underground storage volume, wherein the second pressure that is greater than the first pressure but less than the maximum allowable pressure of the cavern.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates an embodiment of the invention; and

FIG. 2 illustrates an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

It will, of course, be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

This invention claims that to leach an ultra pure cavern an inert gas blanket 102, which could be nitrogen, helium, argon or methane, is injected into the outer annulus string 106 of a salt cavern 101. The blanket pressure is maintained at a pressure above the water injection pressure but less than the maximum pressure for the cavern as defined by the depth of the final cemented casing shoe and the maximum pressure gradient for the cavern.

The blanket depth may be controlled by monitoring the blanket gas pressure and by verification of the blanket depth may be by wire line density measurement for the gas-brine interface. Inert gas may be periodically added to the annulus to maintain the gas-brine interface at the desired depth.

It is further claimed that the geometry of the storage cavern roof may be controlled by the flow of water 103 into the cavern. The water injection flow into the cavern may be maintained between the minimum flow rate of 5 ft/sec velocity and the maximum flow rate of 8 ft/sec. Ideal cavern roof geometry is achieved by flowing at a constant flow rate of approximately between 6 and 7 ft/sec.

Turning to FIG. 1, a cased borehole 112 is located within a salt bed 113. An aqueous liquid 101 is injected into the cased borehole 112 at a first pressure, in order to expose the salt bed 113 to the aqueous liquid 103, thereby dissolving at least a portion of the salt bed and forming a brine solution 104 within an underground storage volume 102. At least a portion of the brine solution 104 is withdrawn 105 from the underground storage volume, while an inert gas 108 is injected into the cased borehole 112 at a second pressure. This provides an inert blanket 109 within the underground storage volume, wherein the second pressure that is greater than the first pressure but less than the maximum allowable pressure of the cavern. The inert gas may be nitrogen, helium, or argon. The inert blanket has a blanket depth, and the blanket depth may be measured by means of a wire line density test.

A length of casing 106, is permanently cemented into the surrounding rock formations 114, with a final cemented casing shoe 111 defining the practical endpoint of the lined casing at an approximate depth (D_(casing)), wherein the maximum allowable pressure (P_(max)) is defined as P_(max)<D_(casing)×G_(max), wherein G_(max) may be equal to 0.85. G_(max) may be equal to0.8. 

What is claimed is:
 1. A method for solution mining comprising; providing an cased borehole located within a salt bed, injecting an aqueous liquid into the cased borehole at a first pressure, in order to expose the salt bed to the aqueous liquid, thereby dissolving at least a portion of the salt bed and forming a brine solution within an underground storage volume, withdrawing at least a portion of the brine solution from the underground storage volume, injecting an inert gas into the cased borehole at a second pressure, in order to provide an inert blanket within the underground storage volume, wherein the second pressure that is greater than the first pressure but less than the maximum allowable pressure of the cavern.
 2. The method of claim 1, wherein the inert gas selected from the group consisting of nitrogen, helium, and argon.
 3. The method of claim 1, wherein the inert gas is nitrogen.
 4. The method of claim 1, further comprising a length of casing, permanently cemented into the surrounding rock formations, with a final cemented casing shoe defining the practical endpoint at an approximate depth (D_(casing)), wherein the maximum allowable pressure (P_(max)) is defined as P_(max)<D_(casing)×G_(max), wherein G_(max)=0.85.
 5. The method of claim 1, further comprising a length of casing, permanently cemented into the surrounding rock formations, with a final cemented casing shoe defining the practical endpoint at an approximate depth (D_(casing)), wherein the maximum allowable pressure (P_(max)) is defined as P_(max)<D_(casing)×G_(max), wherein G_(max)=0.8.
 6. The method of claim 1, wherein the inert blanket has a blanket depth, and the blanket depth is measured by means of a wire line density test. 